1. Field of the Invention
The present invention relates in general to survey operations in an oil and gas wellbore, and in particular relates to techniques for compensating for biased magnetometer measurements in the determination of azimuth.
2. Description of the Prior Art
In order to understand the present invention, it is important to establish the definitions of terminology utilized in wellbore survey operations. Surveys are used to determine the position of a wellbore within the earth. The orientation of the wellbore is determined at a particular depth by the deviation of the wellbore from a predetermined axis. The deviations may be measured with respect to two reference vectors:
(1) the earth's gravitational field vector "G"; and PA1 (2) the earth's magnetic field vector "B". PA1 (1) an axial biasing error e.sub.z ; and PA1 (2) a transverse (or "cross-axial") biasing error e.sub.xy.
In accordance with this convention, G is positive in the vertical downward direction, while B is positive in the north direction and is inclined by a dip angle D below the horizon, as is shown in FIG. 1.
The position of the wellbore relative to the earth's gravitational field vector G is identified as an inclination "I" angle, which is the angle between the longitudinal axis of the drillstring and the earth's gravitational field vector G. The position of the wellbore relative to the earth's magnetic field vector B and the earth's gravitational field G is identified as a magnetic azimuth "A", which is the angle between the horizontal component of the longitudinal axis of the drillstring and the horizontal component of the earth's magnetic field vector B.
Wellbore survey operations are also frequently utilized to determine the dip angle "D", which is the complement of the angle between the earth's magnetic field vector B and the earth's gravitational field vector G (that is, 90 degrees less the angle between vectors B and G). The dip angle D is available in look-up tables, computer programs, and charts for all latitudes and longitudes on the earth's surface.
In conventional wellbore survey operations, accelerometers are utilized to measure the direction of the earth's gravitational field vector G, and magnetometers are utilized to measure the earth's magnetic field vector B. Each vector includes x-axis, y-axis, and z-axis components.
In order to understand the techniques of the present invention for compensating for the magnetic field biasing error "e", it is important first to understand the coordinate systems utilized in surveying operations. FIG. 2 provides a view of the Cartesian coordinate system which is established relative to the bottomhole assembly of a drillstring. The z-axis of the coordinate system is in alignment with longitudinal axis of the bottom hole assembly. The x-axis and y-axis are perpendicular to the z-axis, and are fixed in position with respect to the bottom hole assembly. Therefore, rotation of the bottomhole assembly about the z-axis also rotates the x-axis and the y-axis by an amount measured in terms of tool face angle "T". Note that the inclination angle I provides a measure of the position of the bottomhole assembly of a drillstring relative to the gravity field vector G, and the tool face angle T provides a measure of the angle in the xy-plane between the y-axis and the highside HS of the tool.
During survey operations, magnetometer readings are taken along the three axes to determine the magnetic field vector B components: these measurements are identified as B.sub.x, B.sub.y, and B.sub.z. Accelerometer readings are taken along the three axes to determine the gravitational field vector G components: these measurements are identified as G.sub.x, G.sub.y, and G.sub.z. Since these vectors have both a direction and a magnitude, these tri-axial measurements will have to be scaled, or calibrated, to accurately reflect the magnitude of the vector they represent.
Survey tools utilize these x-y-z arrays of accelerometers and magnetometers to determine the directions and magnitudes of the B and G vectors in the x-y-z frame of reference. This information is used to determine the tool's "attitude", expressed by the inclination angle I, and the azimuth angle A and the tool face angle T.
Once a set of values for B.sub.x, B.sub.y, B.sub.z, G.sub.x, G.sub.y, and G.sub.z are determined for a specific wellbore depth, the azimuth A, inclination I, and tool face angle T of the wellbore at that depth can be determined. Also, the value of the magnetic dip angle D can be calculated. An expected magnetic dip angle D can be looked up in reference tables, computer programs, or charts based on the wellsite's longitude and latitude. The three accelerometers are normally used to find I, T and G, the magnetometers are used to find B, and a combination of measurements from all sensors are required to calculate A and D.
Once the azimuth A and inclination I are determined for the wellbore at a number of specific depths, a directional map of the wellbore can be plotted. This directional map shows how far, and in what direction, the wellbore is deviated from the vertical and magnetic north, and ultimately where the well is bottomed.
In order to provide the greatest accuracy in magnetic field measurements, the wellbore surveying tool is typically housed in a non-magnetic subassembly. Additionally, surrounding subassemblies may also be constructed of a non-magnetic material. However, the drillstring, including these non-magnetic subassemblies, can nonetheless become magnetized during drilling operations, to such an extent that the accuracy of the magnetic field measurements is severely impaired. Any magnetization of the bottomhole assembly in the vicinity of the surveying equipment will introduce a biasing error "e", which is an undesired error component contained in magnetometer readings due to the magnetization of the drillstring. The biasing error includes the following two types of error components:
Since the biasing error e.sub.z and e.sub.xy arise from sources fixed to the subassembly which carries the sensor array, the transverse biasing error e.sub.xy is fixed in position relative to the x-y-z frame of reference and will rotate as the tool is rotated. In other words, the transverse biasing error e.sub.xy will remain invariant as the tool is rotated.
In the prior art, there are a plurality of methods for determining wellbore azimuth A which eliminate or minimize the influence of magnetic field biasing errors, including axial biasing errors e.sub.z, and transverse biasing errors e.sub.xy, but most of the prior art solutions are directed to the minimization or elimination of the axial biasing error e.sub.z. Only the approach of U.S. Pat. No. 4,682,421 to van Dongen and U.S. Pat. No. 4,345,454 to Coles are directed to the minimization or elimination of both the axial biasing error e.sub.z and the transverse biasing error e.sub.xy.
Most of the prior art approaches require either (1) the adoption of an assumption about the value of one component of the earth's magnetic field such as magnetic field strength B, dip angle D, the horizontal component of the magnetic field B.sub.h or the vertical component of the magnetic field B.sub.v, or (2) the adoption of an assumption about the value of two components of the earth's magnetic field in combination with an initial estimation of azimuth which is improved through iterative calculations performed upon survey data, with the goal of converging upon a correct solution. These iterative approaches require additional processing resources, slow down the determination of wellbore orientation parameters such as azimuth A, and are subject to convergence to an incorrect or inadequate conclusion. All of the iterative processes require a good initial estimate in order to provide good survey data.